Water-resistant gypsum products and method of making



Patented a: 17, I950 s PATENT OFFICE WATER-RESISTANT GYPSUM PRODUCTS- .AND METHOD OF MAKING Thomas P. Camp, Arlington Heights, 111., assignor I to United States Gypsum Company, Chicago,

Ill., acorporation of Illinois N o Drawing.

1 The present invention relates to' water resistant gypsum products and to a process'for preparing the same.

Ordinary gypsum wallboard, gypsum tile, gypsumblock, gypsum casts, and thelike have very little resistance to water. Whenordinary gypsum .wallboard, for example, is immersedin Water, the board quickly absorbs a considerable amount of water, .and loses a great deal of its strength. Actual tests have demonstrated that when a 2" x 4" cylinder of gypsum board core material was immersed in water at about 70 F., the cylinder showed a water absorption of after minutes immersion. Many attempts have been madein 'the past to improve the water resistance of gypsum products. These attempts'have included the incorporation of water resistant material such as metallic soaps, as-

phalts, waxes, resins, etc., within the calcium sulfate hemi-hydrate slurry. They have also included attempts to coat the finished gypsum product with water-resistant films or coatings. One specific example of past attempts to Waterproof gypsum integrally by the addition of wa ter-repellent substances is disclosed in Patent No. 2,198,776 to King and Camp. This shows the incorporation of paraffin wax, asphalt, etc, into the aqueous slurry by spraying the molten material into the slurry.

A -further improvement, in which a combination of asphalt and wax is used in certain critical proportions, is described in applicants colpending application Serial No. 585,791, filed March 30, 1945; "now Patent No. 2,432,963. That method comprises the addition to the aqueous plaster slurry of an emulsion of wax, suchas paraiiin wax, and asphalt in the relative proportions of from about one part to about ten parts of asphalt per part of wax. Since the asphaltis a relatively poor solvent for wax at ordinary temperatures the solution formed at high temperatures tends on cooling to deposit microscopic wax crystals on the asphalt-wax surface,wherebyunusual water-repellent properties are secured. I

have found that water forms a negative-meniscus with theasphalt-wax'surfaces described in'said copending applications.

It is the object of the present invention to effect further improvements in the preparation of waterproofgypsum products, and a further 010-- Application November 16, 1946, 'Serial N0. 710,394"

v 8'Cla ims. (CL-10641716) 2 ject to effect these improvements in a simple fashion and with inexpensive materials.

Other objects will become apparent from the ensuing description of the invention.

I have found that these and other desirable objects may be achieved by the addition of certain materials in minor amounts to the plaster slurry-emulsion combination. Specifically I have found that the addition of small quantities of a substance containing an aluminate, such as sodium aluminate, calcium aluminate or Portland cement, increases the waterproofing efiect of the emulsion tremendously. This is a completely unexpected and surprising result. WhilePortland cement is known to accelerate the hydration of plaster, it could not be predicted that its use would lead to more eifective waterproofing, partioularly-since other Well known accelerators have no such effect. Moreover, variations in the 'effectiveness of various asphalt-wax emulsions are A series of 2";x 4" cylinders was prepared from'commercial plaster (calcium sulfate hemihydrate) and a commercial. protein-stabilized wax asphalt emulsion which contained 5.25 parts of asphalt per part of wax. The wax was a paraf fin wax having a melting point of about 122-l24 F. and the asphalt had a ring--and-ball soften--v ing point of about F. Waxes melting up to F. and asphalts melting up to F. may

be used. Sufficient emulsion to provide 80 grams of wax-asphalt mixture was diluted with 900 c. 0. water. To this dilutedemulsion were added 1600 grams of calcined gypsum plus the materials indicated in the following table' The masswas" mixed'tho'roughly an'd'bOO c. c. of a foam were added. This foam was prepared by stirring rapidly (as for instance in a malted milk mixer) 14 c. c. of 20% rosin soap dissolved in 175 c. c. of water at 70 F. until a foam volume of 900 c. 0. had been reached. The amount of foam as above stated was stirred into the slurry already containing the emulsion and the slurry was then poured into molds to form 2" x 4 cylinders. After the plaster had set, the cylinders were dried in a circulating air oven at a temperature suflicient to dry the casts without recalcination of the gypsum, but hot enough to melt the asphalt-wax composition therein. About at least 150 F. is required. The dry cylinders were weighed and immersed in water at 70 F. At intervals they were removed, reweighed to determine amount of water absorbed, and replaced in the water bath. The results of the tests are given in the following table, together with the materials added with the plaster and the amounts cement containing a higher percentage of alumina than ordinary Portland cement. The amount of the aluminate-containing substance is small, and need not exceed about 2% on the basis of the calcined gypsum present.

The cylinders containing terra alba (which is ground calcium sulfate dihydrate) are typical of the products secured when omitting the aluminate-containing substance. The marked reduction in water-absorption is plainly evident.

Just what causes the remarkable increase in the effectiveness of the wax-asphalt waterproofing composition when one of the above substances is used is not precisely known, but it may be due, at least in part, to a modification of the crystal form of the calcium sulfate dihydrate formed, or to an improvement in the surface characteristics of the crystals so that they are more readily and/or completely coated by the wax-asphalt material. The result appears to be due to a change in the size of the crystals, in the direction of increased thickness and shorter length, whereby there will be a very marked decrease in the total surface area per given weight of gypsum crystals, so that a given amount of the waterproofing material will be more effective because there is less surface to be covered. It is well known when using an accelerator such as terra alba that while the speed of setting is greatly increased, the formed gypsum crystals are very fine, long and thin, and hence have a large total surface. But when one of the aluminates (including Portland cement) is present, the crystals are relatively short and thick. This can readily be observed under a microscope. There thus is a decided advantage in the conjoint use of the accelerator '(the terra alba) and a crystal structure modifier (thealuminate-yielding or aluminate material), as by utilizing the former to get a quicker set, yet the crystals formed are large enough to' enable either the use of a lesser amount of the waterproofing material or a better utllization of the amount added. But, irrespective of 4 any theory, the results are manifest and important.

These results indicate strikingly the unexpected improvement conferred by my invention, even when used in conjunction with other conventional accelerators which confer no improvement in water resistance.

While the preferred method of this invention involves the use of a ready-made wax-asphalt emulsion which has been dispersed by a protein material, the invention is by no means restricted thereto. One may use other suitable emulsifying materials in conjunction with or as a replacement for proteins, such as soaps, various wetting agents such as the sulfated fatty alcohols, amine soaps, ether-alcohols, substituted sulfonates, various starch and dextrine materials, and the like, as will be obvious to those skilled in the art of emulsions. Many methods of preparing emulsions are described in the literature. Among the publications on the subject are W. Clayton's Emulsions and Emulsificationfi Harry B. Weisers Colloid Chemistry, and Berkman and Egloffs Emulsions and Foams.

It is also within the ambit of the invention to prepare emulsions of wax and asphalt separately and to add the requisite quantity of each at the time of use.

As disclosed in the hereinbefore referred to copending application, my preferred proportions of wax and asphalt are from about one part to about ten parts of asphalt per part of wax. I prefer to use between about 2% and about 15% of the wax and asphalt mixture based on, the weight of the solids contents of the entire mixture.

To obtain the optimum waterproofing, I prefer to heat my gypsum products to a temperature above the melting point of the wax-asphalt mixture in order to obtain the best possible coating on the surface of the gypsum crystals. On cooling, the wax crystallizes out of the asphalt and provides the unique water-repellent effect.

I claim:

1. A water resistant gypsum product comprising a set mass of interlaced gypsum crystals containing from about 2% to about 15% of a therein deposited mixture of asphalt and paraffin wax in the relative proportions of from about one part to about ten parts of asphalt per each part of wax and a small amount, not exceeding about 2-% of a substance selected from the group consisting of alkali and alkaline earth aluminates and Portland cement sufliicient markedly to increase the water-resistance of the product as compared with one devoid of said substance.

2. A water-resistant gypsum product as defined in claim 1 in which the substance is sodium aluminate.

3. A water-resistant gypsum product as defined in claim 1 in which the substance is calcium aluminate.

4. A water-resistant gypsum product as defined in claim 1 in which the substance is Portlan cement.

5. Process of producing a water-resistant gypsum product which comprises adding to a calcined ypsum slurry an aqueous asphalt-parafiin wax emulsion, in which the ratio of asphalt to wax is from about one to ten parts of asphalt to each part of wax, in an amount of from about 2% to about 15% on the weight of the gypsum in the product, together with not exceeding about 2% of a substance selected from the group consisting of alkali and alkaline earth aluminates and Portland cement sufficient markedlyto improve the water-repellent properties of said product as compared with one devoid of said substance.

6. The process as defined in claim 5 in which the substance is sodium aluminate.

'7. The process as defined in claim 5 in which the substance is calcium aluminate.

8. The process as defined in claim 5 in which the substance is Portland cement.

THOMAS P. CAMP.

REFERENCES CITED The following references are of record in the file of this patent:

Number 6 UNITED STATES PATENTS Namev Date King Apr; 20, 1937 King et al Apr. 30, 1940 Hann Aug. 27, 1940' Koenig Aug. 4, 1942 Wiss Feb. 1, 1944 Watts Apr. 30, 1946 Camp Dec. 16, 1947 OTHER REFERENCES Eckel:v Cements, Limes and Plasters,':3rd ed., 1928, pp. 60-61. 

1. A WATER RESISTANT GYPSUM PRODUCT COMPRISING A SET MASS OF INTERLACED GYPSUM CRYSTALS CONTAINING FROM ABOUT 2% TO ABOUT 15% OF A THEREIN DEPOSITED MIXTURE OF ASPHALT AND PARAFFIN WAX IN THE RELATIVE PROPORTIONS OF FROM ABOT ONE PART TO ABOUT TEN PARTS OF ASPHALT PER EACH PART OF WAX AND A SMALL AMOUNT, NOT EXCEEDING ABOIUT 2% OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH ALUMINATES AND PORTLAND CEMENT SUFFICIENT MARKEDLY TO INCREASE THE WATER-RESISTANCE OF THE PRODUCT AS COMPARED WITH ONE DEVOID OF SAID SUBSTANCE. 